One practice which may be employed when drilling a borehole is to enlarge a hole with a reamer. A reamer may be constructed to have a fixed diameter, in which case the reamer must start cutting at the surface or at the end of an existing hole of equal or greater size. Alternatively a reamer can be constructed so as to be expandable so that it can enlarge a borehole to a greater diameter than that of the hole through which the (unexpanded) reamer was inserted.
Enlarging a borehole with a reamer may be done as a separate operation to enlarge an existing borehole drilled at an earlier time. Enlarging with a reamer may also be done at the same time as using a bottom hole assembly which has a drill bit at its bottom end. The drill bit makes an initial hole, sometimes referred to as pilot hole, and a reamer positioned at some distance above the drill bit increases the hole diameter.
There is more than one type of reaming tool. Some reamers are constructed to be eccentric, relative to the drill string to which they are attached and the borehole which they are enlarging. Other reamers are constructed to remain concentric with the drill string and the borehole. These different types of reamers tend to be used in different circumstances. There are many instances where concentric reamers are the appropriate choice.
A reamer may have a plurality of cutter assemblies, each comprising a support structure with attached cutters, arranged azimuthally around the axis of the tool. In the case of an expandable reaming tool it is common to have a plurality of radially expandable support elements bearing cutters positioned around the axis of the tool. Often the tool has three such cutter assemblies which extend axially and are arranged at 120° intervals azimuthally around the tool axis. A mechanism is provided for expanding these cutter assemblies radially outwardly from the axis and this mechanism typically uses hydraulic pressure to force the support structures of the cutter assemblies outwardly.
This tool construction has commonly been used for concentric reamers. In some constructions, each of the individual cutter assemblies arranged around the tool axis is an assembly of parts attached together so as to move bodily as one piece, in which case the assembly is often referred to as a “block” (one part of this assembly may be a shaped monolithic block) although the term “arm” has also been used for such an assembly. The individual cutter assemblies (i.e. individual blocks) may be moved outwards in unison by one drive mechanism acting on them all, or may be moved outwards by drive mechanism(s) which does not constrain them to move in unison.
Cutters attached to the supporting structure may be hard faced and may be PDC cutters having body with a polycrystalline diamond section at one end. The body may be moulded from hard material such as tungsten carbide particles infiltrated with metallic binder. The polycrystalline diamond section which provides the cutting part may then comprise particles of diamond and a binder. In many instances, the polycrystalline diamond section is a disc so that the hardest end of a cutter is a flat surface but other shapes can also be used.
Reamer designs customarily position at least some cutters with their cutting faces at the leading face of a support structure and with the cutters projecting radially outwardly from the support structure. The parts of the cutter which project outwardly beyond the support structure may be the parts of the cutter principally involved in cutting as the rotating reamer is advanced and/or as an expandable reamer is expanded.
The greatest radius swept by a reamer (so-called full gauge) may be the radial distance from the axis to the extremity of the outermost cutter(s). In order to position a reamer centrally in the reamed bore, it is customary for a supporting structure to include a section which does not include cutters but has a so-called gauge pad (alternatively spelt “gage pad”) which is a surface positioned to confront and slide on the wall of the reamed bore. In an expandable reamer, it is known to position gauge pads at a radius which is slightly less than full gauge so as to facilitate cutting during the period when the reamer is being expanded.
It is desirable that a reamer maintains stable cutting behaviour, centred on the axis of the existing bore, even though it has significant mass of collars and other drill string components placed above and/or below it. Yet frontal area in frictional contact with the formation, which helps to dampen oscillations, is smaller than with a drill bit of the same diameter. It has been observed that reamers tend to be more prone to the phenomenon of whirling than are drill bits. In this context, whirling refers to a motion in which the tool axis moves around a centre line rather than staying on it, leading to a mis-shaped or oversized borehole.